The primary objective of this study is to assess the incremental clinical activity from radiolabeling Anti-B1 with 131 Iodine as compared to unlabeled Anti-B1 inpatients with relapsed low-grade B-cell NHL in a randomized controlled trial. Secondary objective includes comparison of the response rates, duration of overall and complete response, time-to-treatment failure, and safety and tolerance between the unlabeled Anti-B1 arm and the Anti-B1 RIT arm. In additions, these comparisons will be made between the unlabeled Anti-B1 arm and crossover RIT therapy.